1. Field of the Invention
The present invention relates to a toroidal type continuously variable transmission. More particularly, the invention relates to a dual cavity type toroidal type continuously variable transmission, the supporting mode of which is improved with respect to the input shaft of discs on the input side. The toroidal type continuously variable transmission of the invention can be used as a speed change gear for an autovehicle or for various kinds of industrial machinery, for example.
2. Related Background Art
The use of a toroidal type continuously variable transmission which is schematically shown in FIG. 3 and FIG. 4 is under study as a speed change gear for an autovehicle. This toroidal type continuously variable transmission is such that a disc 2 on the input side is supported coaxially with an input shaft 1 which is supported rotatably inside the case of a speed change gear. Likewise, a disc 4 on the output side is fixed to the end portion of an output shaft 3 which is rotatably supported inside the case of the speed change gear. On the inner surface of the case having the toroidal type continuously variable transmission housed in it or on the supporting bracket provided inside the case of the speed change gear, trunnions 5 and 5 are swingably arranged centering on the pivots in the torsional positions with respect to the input shaft 1 and output shaft 3.
Each of the trunnions 5 and 5 is formed by a sufficiently rigid metallic material, and the aforesaid pivots are provided on the outer faces of both ends. Also, on the circumference of the displacement shafts 6 and 6 arranged in the central portion of each of the trunnions 5 and 5, the power rollers 7 and 7 are rotatively supported, respectively. Then, each of the power rollers 7 and 7 is pinched between the discs 2 and 4 on the input and output sides.
On each one face of the discs 2 and 4 on the input and output sides in the axial direction, where these discs face each other, the concave face 2a on the input side and the concave face 4a on the output side are formed, each section of which represents a circular toroidal curve centering on the point on the center line of the aforesaid pivot, respectively. Then, the circumferential faces 7a and 7a of the power rollers 7 and 7, each of which is formed to be a convex face providing a rotary circular surface, are arranged to abut, respectively, the aforesaid concave face 2a on the input side and concave face 4a on the output side.
Between the input shaft 1 and the disc 2 on the input side, a pressure device 8 of a loading cam type is arranged. By this pressure device 8, the disc 2 on the input side is pressed toward the disc 4 on the output side. The pressure device 8 comprises a cam board 9 which rotates together with the input shaft 1, and a plurality of rollers 11 and 11 (four rollers, for example) supported by a holder 10. On one side face (on the right side face in FIG. 3 and FIG. 4) of the cam board 9, a first cam surface 12 is formed to present concave and convex surfaces in the circular direction. At the same time, on the outer face (the left side face in FIG. 3 and FIG. 4) of the disc 2 on the input side, a second cam surface 13 is formed in the same configuration. Then, the aforesaid plural rollers 11 and 11 are arranged rotatively around axes extending in the radial direction with respect to the center of the input shaft 1. In this respect, the disc 2 on the input side can move slightly in the axial direction (the left and right directions in FIG. 3 and FIG. 4). This disc is also supported rotatively in the rotational direction.
With the rotation of the cam board 9 along with the rotation of the input shaft 1, the difference in rotational phase occurs with respect to the disc 2 on the input side. Then the plural rollers 11 and 11 are caused to ride on the first cam surface 12 and the second cam surface 13, thus causing the cam board 9 and the disc 2 on the input side to part from one another. Since the cam board 9 is supported by the input shaft 1, which is connected by a bearing to the case of the speed change gear, so as not to allow the board to move in the axial direction, the disc 2 on the input side is pressed toward the power rollers 7 and 7 while the power rollers 7 and 7 are pressed toward the disc 4 on the output side. The disc 4 on the output side is connected to the case of the speed change gear to be only rotative together with the output shaft 3, but not to be movable in the axial direction. As a result, the power rollers 7 and 7 are pressed between the disc 2 on the input side and the disc 4 on the output side. By the application of this pressing force, a pressure is generated between each of the circumferential faces of the power rollers 7 and 7 and the concave faces 2a and 4a on the input and output sides, respectively. Thus the rotation of the disc 2 on the input side is transmitted to the disc 4 on the output side through the power rollers 7 and 7 with almost no slippage. The output shaft 3 fixed to the disc 4 on the output side rotates accordingly.
When the rotating speed ratio (transmission ratio) between the input shaft 1 and output shaft 3 is changed and deceleration is performed at first between the input shaft 1 and output shaft 3, each of the trunnions 5 and 5 should swing about the respective pivot as shown in FIG. 3 to incline each of the displacement shafts 6 and 6 so that the circumferential faces 7a and 7a of the power rollers 7 and 7 abut respectively upon the portion close to the center of the concave face 2a on the input side and the portion close to the outer circumference of the concave face 4a on the output side. On the contrary, when an acceleration is performed, it is possible to obtain an intermediate transmission ratio between the input shaft 1 and output shaft 3 by swinging the trunnions 5 and 5 as shown in FIG. 4 to enable each of the displacement shafts 6, 6 to be inclined so that the circumferential faces 7a, 7a of the power rollers 7 and 7 abut respectively the portion close to the outer circumference of the concave face 2a on the input side and the potion close to the center of the convex face 4a on the output side. If the inclination angle of each displacement shaft 6 is set between those of FIG. 3 and FIG. 4, the intermediate transmission ratio between the input shaft 1 and the output shaft 3 may be obtained.
The fundamental structure and function of a toroidal type continuously variable transmission is as described above. When a toroidal type continuously variable transmission of this kind is utilized as a speed change gear for an auto-vehicle having an engine whose output is great, two pair of discs 2 and 4 are installed, and arranged in parallel to each other in the direction of power transmission. This has been known, for example, as disclosed in Japanese Patent Laid-Open Application Nos. 62-258255, 2-163549 and 4-69439. Of those known transmissions, FIG. 5 represents the structure disclosed in the Japanese Patent Laid-Open Application No. 4-69439.
In the structure thus disclosed, an input shaft 15 is supported only rotatively inside a housing 14. The input shaft 15 comprises a front half 15a which is coupled to the output shaft of a clutch, and others, and a rear half 15b which is made slightly rotative with respect to the front half 15a. On both ends of the rear half 15b in the axial direction (left and right directions in FIG. 5), a pair of discs 2 and 2 on the input side are supported through ball splines 16 and 16 in a state that convex faces 2a and 2a themselves are allowed to face each other. Also, on the back sides (opposite sides of the convex faces 2a and 2a on the input side in the axial direction) of the discs 2 and 2 on the input side, recesses 20 and 20 are formed in the central portion. Then disc springs 30 and 30 are provided respectively between the rear surfaces of the recesses 20 and 20 and a loading nut 28 (in the case of a recess 20 on the right-hand side in FIG. 5) or a loading plate 29 (in the case of a recess 20 on the left-hand side in FIG. 5). By each of these disc springs 30 and 30, a preliminary pressure is exerted on each of the discs 2 and 2 on the input side toward each of the discs 4 and 4 on the output side, which will be described next.
In the circumference of the intermediate portion of the aforesaid rear half 15b, a pair of discs 4 and 4 on the output side are rotatively supported with respect to the input shaft 15 in a state that each of the concave faces 4a and 4a on the output side is arranged to face each of the concave faces 2a and 2a on the input side. Also, the plural power rollers 7 and 7, which are rotatively supported by the plural trunnions through the displacement shaft 6 (FIG. 3 and FIG. 4), are pinched between each of the concave faces 2a and 4a themselves on the input and output sides. Each of the power rollers 7 and 7 incline in synchronism in order to make the transmission ratio agreeable for each of the discs 2 and 2 on the input side and the discs 4 and 4 on the output side.
Also, on the aforesaid front half 15a and the opposite portion inside the housing 14, the output shaft 17 is arranged coaxially with the rear half 15b of the input shaft 15, and is rotatively supported independent of this rear half 15b. Then, between this output shaft 17 and the pair of discs 4 and 4 on the output side, means for transmitting rotation, which will be described later, is installed to make the rotation of the discs 4 and 4 on the output side transmittable to the output shaft 17.
Inside the housing 14, a separation wall 18 is arranged on a portion between the pair of output discs 4 and 4, and then, a circularly tubular sleeve 21 is supported by a pair of roller bearings 27 and 27 inside the through hole 19 which is provided for this separation wall 18. The pair of discs 4 and 4 on the output side are coupled to both ends of this sleeve 21 by means of splines. In other words, the male splines formed on each outer circumference of both ends of the sleeve 21 are arranged to engage with the female grooves formed on each inner circumference of the discs 4 and 4 on the output side, respectively. Also, in the interior of the separation wall 18 on the intermediate portion of the sleeve 21, a first gear 22 is fixedly installed. Further, roller bearings 35 and 35 are arranged on a part of each of the discs 4 and 4 on the output side between the inner circumference of the extrusion of the sleeve 21, and the outer circumference of the input shaft 15. Each of these roller bearings 35 and 35 is arranged to allow correlated rotation and correlated displacement in the axial direction between the input shaft 15 and each of the discs 4 and 4 on the output side.
Meanwhile, inside the housing 14, a transmission shaft 23 is rotatively supported in parallel to the input shaft 15 and output shaft 17. Then an arrangement is made so that a second gear 24 fixed to one end of this transmission shaft 23 (left-hand end in FIG. 5) and the first gear 22 engage with each other directly, and that a third gear 25 fixed to the other end of the transmission shaft 23 and a fourth gear 26 fixed to the end of the output shaft 17 engage with each other through an idler gear which is not shown. With such means for transmitting rotation, the output shaft 17 rotates in the direction opposite to that of the discs 4 and 4 on the output side upon rotation of the pair of discs 4 and 4 on the output side.
Further, between the front half 15a and one (left-hand side in FIG. 5) of the discs 2 and 2 on the input side, a loading cam type pressure device 8 is installed to make it possible for this disc 2 on the input side to be freely pressed in the axial direction toward the disc 4 on the output side, which faces this disc 2 on the input side, upon rotation of the input shaft 15.
When operating the toroidal type continuously variable transmission shown in FIG. 5, which is structured as described, the pair of discs 2 and 2 on the input side rotate simultaneously upon rotation of the input shaft 15. This rotation is transmitted to the pair of discs 4 and 4 on the output side simultaneously with the same transmission ratio, thus the rotation is transmitted to the output shaft 17 by the aforesaid means for transmitting rotation. At this juncture, since the transmission of the rotational force is performed separately in the two systems arranged in parallel to each other, it is possible to make a large power (torque) transmittable. Also, by the function of the pressure device 8, the gap between the pair of discs 2 and 2 on the input side themselves tends to be narrowed when the transmission is in operation. As a result, the concave faces 2a and 2a on the input side of the discs 2 and 2 on each input side, the concave faces 4a and 4a on the output side of the discs 4 and 4 on each output side, and the circumferential faces 7a and 7a of each of the power rollers 7 and 7 are caused to abut strongly to effectuate the power transmission, efficiently. In this respect, the structure disclosed in the Japanese Patent Laid-Open Application No. 2-163549 is essentially the same as the structure shown in FIG. 5. The structure disclosed in Japanese Patent Laid-Open Application No. 62-258255 deals more with its principle and is not specific as compared with the structure shown in FIG. 5.
Now, in the case of the toroidal type continuously variable transmission which is structured to function as described above, it is desirable to improve the various aspects given below in order to reduce its costs of manufacture while securing sufficient performance and durability. In other words, for a stabilized operation of a toroidal type continuously variable transmission, it is necessary to position on the central axis of the input shaft 15 the virtual center (rotational center of each of the concave faces 2a and 2a on the input side, which presents a rotationally circular surface) of the concave faces 2a and 2a of the discs 2 and 2 on the input side, that is, the respective surface of toroidal curve. If this virtual center is displaced from the central axis even slightly, each of the concave faces 2a and 2a on the input side is caused to move eccentrically to the extent that it is displaced. If such an eccentric motion takes place, the contacting state (abutting pressure between both surfaces) of the circumferential faces 7a and 7a of the power rollers 7 and 7, and the concave faces 2a and 2a on the input side changes minutely, respectively, and then, not only vibrations occur on each of the abutting portions between both faces 7a and 2a, but the transmission efficiency of driving force is lowered between these faces 7a and 2a.
Here, in the conventional structure described above, the discs 2 and 2 on the input side are supported by the ball splines 16 and 16 on the input shaft 15, respectively. Therefore, in order to make the virtual center and the center axis agreeable, it is necessary to secure the exact precision of the ball splines 16 and 16. The dimensional errors of the female spline grooves on the inner circumferences of the discs 2 and 2 on the input side, and the male spline groves on the outer circumference of the input shaft 15 result directly in the disagreement between the virtual center and the central axis. Particularly, the dimensional precision (coaxiality) of the concave faces 2a and 2a on the input side formed on the discs 2 and 2 on the input side, as well as the female spline grooves, must be maintained exactly. Therefore, the machining of the discs 2 and 2 on the input side is extremely difficult (because of the required high precision), hence inevitably leading to the high manufacturing costs of the discs 2 and 2 on the input side. A toroidal type continuously variable transmission of the present invention is designed in consideration of these aspects.